FWA70, IP Passthrough, and IPv6 PD

I am attempting to configure my FWA70 to pass traffic as directly as possible to my router. I need to have not only IPv4 address but also the IPv6 RA / DHCPv6 as well as the IPv6 PD (Prefix Delegation) that my provider assigns.

When configured as a router it does use the PD to provide the LAN with a subnet, but I need to use that for the lan behind my main router. The documentation seems to say that passthrough would allow this but it absolutely does not.

I am using a fixed passthrough. When I disable DHCP and IPv6 on the "LAN" neither the public IPv4 address nor any IPv6 address comes through DHCP or RA. When I enable though, though, IPv4 gets a public IP address seemingly shared with the router and an address from the PD pool. I need the PD subnet to be given to my router, though, so IT can delegate it. It should be getting an address from the RA of the upstream provider

How can I get this to work? I've been trying different combinations of settings for literal hours and it seems impossible. My APN is set manually, I can get traffic to pass via IPv4 and IPv6 but only when the FWA70 is acting as the router, so at least that works but it won't work with my main router.

I need the FWA to just get out of the way and let me talk to the provider